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Arc Flash
Description Arc Flash is the result of a rapid release of energy due to an arcing fault between a phase bus bar and another phase bus bar, neutral or a ground. During an arc fault the air is the conductor. Arc faults are generally limited to systems where the bus voltage is in excess of 120 volts. The air itself provides a high resistive path for the arc, which makes it more difficult to clear quickly than a 3 phase bolted fault with overcurrent elements. There are two components to an arc flash, a physical and radiative. During an arc flash event the heat is so intense that it causes an explosion (arc blast) as nearby metal expands 50,000+ times its volume as it changes from a solid to a plasma. The metal plasma also radiates an incredible amount of infrared and ultraviolet light, which is absorbed by anything around it. Arc flash studies try to determine the worst case amount of energy release, measured in cal/cm^2. This is used to determine the personal protective equipment (PPE) that is required. Causes Inadvertent contact Loose connections Insulation failure Poorly maintained equipment Voltage transients Unsuccessful Short Circuit Interruption Animals (squirrels, snakes .. Etc) Mitigations Operational Fixes Denergize Equipment Denerergize equipment during maintenance and remove any risk of an arc flash event. Create Space Use robotics or remote racking equipment to increase the space between personal and the equipment. Reduce Available Fault Current Operationally reconfigure the system, use current limiting fuses, circuit breakers, or chokes to reduce the available fault current. The severity of an arc flash is measured in the amount of energy released. Depending on how the system is setup, a low impedance arc may release less energy (I^2*T) than a high impedance arc with lower fault current due time to trip being longer. Reducing available fault current doesn't guarantee lower calculated maximum arc flash energies. Also, a high impedance grounded system can naturally reduce the available ground fault current below what is required to sustain a ground arc flash event. It won't ,though, provided protect for phase-phase and 3 phase arc flash events, which is worse case in the IEEE 1584 calcs. This won't change the required PPE but it would reduce the risk of an arc flash event. 'Arc Flash Testing Circuit Breaker Time Over Current Curves' The tripping times using standard curves my the manufacture are often conservative and testing or purchasing tested breakers might reduce the calculated PPE required over using standarded curves. The IEEE IAS magazine issue quoted below mentions this as well as this Eaton document . Relay Settings and Schemes Flash Detection This type of method operates on the principle that if light is picked up inside of the switchgear by a fiber-optic detector and current above the threshold is detected, an arc flash event has occurred and tripping should occur without delay. Operates very fast and in as little as 0.25 cycles. SEL-751 and SEL-751A Relays Siemens 7XG3124 - ReyArc24 - Another product that operates the same way SEL's does. ABB REA system - operates the same as SEL's. Relay Schemes Schemes that are not time coordinated can provide quicker clearing. High and Low Impedance Bus Differential - It can operate in less than 1.5 cycles. Requires additional relay, dedicated CTs, wiring installation. Testing more complex. Does not operate for faults outside differential zone. Zone interlocking (ZSI) - It can operate in 3-5 cycles. A poor man's differential. It requires communications. Maintenance Switch Use a maintenance switch or push buttons on the relay to speed up the tripping at the expense of security. The new coordinated settings should trip instantly for any detected fault. This method can easily be retrofitted to an existing arrangement with electronic relays. Re-coordinate the system Review the system and determine if the over-current curves can be tightened up to reduce the maximum amount of energy released before the fault is cleared. If electronics relays are being used, coordination time intervals of 0.25 may be used and lower still with testing and analysis. Equipment Arc Flash Resistant Switchgear Use arc flash resistant switchgear that is designed to limit the exposure and/or redirect the energy through plenums or arc flaps. Eaton Arc Resistant Switchgear Siemens Arc Resistant Switchgear Schneider Arc Resistant Switchgear Crowbar System Use a system that crowbars the three phases when it detects light and current. This must be near the switchgear and may damage nearby equipment. Some systems create a parallel arc concealed inside of a drum to crowbar the system. Tested Circuit Breakers Using arc flash tested circuit breakers will base the clearing times of the breakers on actual test data instead of conservative generic curves. Documents University of Idaho Lecture Notes on Arc Flash University of Idaho Handout :Intro to Arc Flash University of Idaho Handout: Arc Flash Table IEEE 1584 - Guide for Performing Arc Flash Hazard Anaylsis Michigan Tech Arc Flash Presentation IEEE Industry Applications Magazine - Issue 4 July/Aug 2011 - This entire issue was dedicated to arc flash and arc flash mitigation techniques. This is a can be gotten through IEEE Xplorer (http://ieeexplore.ieee.org/Xplore/home.jsp) with membership through IEEE Industrial Application Society. IAS is good in general for arc flash information. GE Pros and Cons Table of Different Mitigating Methods - same table as in the above IEEE IAS magazine issue IEEE Industrial Application Magazine - Issue 6 August 2010 - Arc Flash Calculations for Exposures to DC Systems - Calculations for arc flashes on DC systems. Excel Calculators EPowerPlus Calculator I-Gard Calculator Mike Holt Calculator Software ETAP SKM CAPE